First, ensure you have reviewed and actioned all promotion errors before attempting to start the promotion.
There are two ways to start a promotion:

1. A promotion will automatically start overnight on the Start Date entered when the promotion was created.

a. If you need to adjust the Start date, highlight the promotion and click Change

The Promotion maintenance - Change window is dispalyed.

b. Adjust the Start Date and click Next.

The Processing window is displayed.

c. Click Cancel to return to the Promotion Enquiry - New Promotions window.

Or, 

2. It can be manually started.

a. Highlight the promotion and click Start and the promotion products will immediately scan at the promotional price.

c. Click Cancel to close the Promotions Enquiry window.